Lately, a hybrid vehicle, which has an internal combustion engine and a motor generator(s) as drive sources of the vehicle, attracts attention because of increased public demands of low fuel consumption and low exhaust emissions. For example, JP H07-135701A teaches a hybrid vehicle, which has an internal combustion engine and first and second motor generators. A drive force of the engine is divided to two systems through a planetary gear mechanism. An output of one of the systems is used to drive a drive shaft to drive wheels of the vehicle. Furthermore, an output of the other one of the systems is used to drive the first motor generator to generate an electric power. The electric power generated by the first motor generator and/or electric power supplied from a battery is used to drive the second motor generator to enable driving of the drive shaft with the power supplied from the second motor generator.
In the hybrid vehicle, which has the engine and the two motor generators, it is demanded to achieve three objectives, i.e., (1) controlling of the rotational speed of the engine, (2) controlling of the output torque and (3) limiting of input and output of the electric power at the battery. However, in the system, in which the engine shaft (an output shaft of the engine) and the drive force output shaft are coupled with each other through a drive force transmission arrangement, which has, for example, the planetary gear mechanism(s), when the two motor generators are individually controlled without integrally controlling the two motor generators, the above-specified three objectives may not be achieved, or the control operation of the motor generators become complicated. For instance, in a system, which has two planetary gear mechanisms, the above-specified three objectives may not be achieved. Also, in a system, which has a single planetary gear mechanism, the control operation of the motor generators may become extremely complicated.